Functional fluids with servo valve erosion resistance

ABSTRACT

Phosphate ester based functional fluids containing novel anti-erosion additives provides enhanced results in erosion control.

[0001] This application claims benefit of Provisional U.S. Serial No. 60/285,105 filed Apr. 20, 2001.

FIELD OF THE INVENTION

[0002] This invention relates to phosphate ester fluids used in transmitting power in aircraft hydraulic systems. More specifically it relates to enhancing the anti-erosion properties of such fluids.

BACKGROUND OF THE INVENTION

[0003] Functional fluids are used in a wide variety of industrial applications. For example they are used as the power transmitting medium in hydraulic systems, such as aircraft hydraulic systems.

[0004] Functional fluids intended for use in aircraft hydraulic systems must meet stringent performance criteria such as thermal stability, fire resistance, low susceptibility to viscosity changes over a wide range of temperatures, good hydrolytic stability, elastomer compatibility and good lubricity.

[0005] Organic phosphate ester fluids have been recognized as a preferred fluid for use as a functional fluid such as in hydraulic fluids. Indeed, in present commercial aircraft hydraulic fluids phosphate esters are among the most commonly used base stocks.

[0006] As with other functional fluids, organic phosphate ester based fluids require the incorporation of various additives to enhance the performance of the fluid. For example, experience has shown that orifices in the servo control valves of aircraft hydraulic systems are subject to erosion which is attributed to streaming current induced by fluid flow. Valve orifice erosion, if extensive, can greatly impair the functioning of the valve as a precise control mechanism. Therefore various additives have been used in functional fluids as erosion inhibitors. Nonetheless, there remains a need for increased choice of useful erosion inhibitors, especially for improved erosion inhibitors.

[0007] One object of the present invention is to provide phosphate ester based aircraft hydraulic fluids with enhanced anti-erosion properties.

SUMMARY OF THE INVENTION

[0008] Briefly stated a phosphate ester functional fluid, especially a hydraulic fluid, is enhanced by incorporating in the fluid an erosion inhibiting an erosion inhibiting amount of an additive or mixture thereof having the formula

RSO₃M

[0009] where M is preferably an alkali metal such as lithium, sodium, potassium, cesium. It could also be ammonium (NH₄). The R group consists of a perfluorinated hydrocarbyl group from 1 to 12 carbon atoms. Preferred are the C1 to C8 carbon atoms. The said hydrocarbyl group can be linear or branched. Example are metal salts of 1,1,2,2,3,3,4,4,4-nonafluorobutane-1-sulfonic acid. The preferred M is potassium.

DETAILED DESCRIPTION OF THE INVENTION

[0010] The anti-erosion properties of phosphate ester based functional fluids, especially aircraft hydraulic fluids, are enhanced by adding to the fluid an effective amount of a salt or mixture of salts represented by the formula

RSO₃M

[0011] where M is an alkali metal such as lithium, sodium, potassium, cesium. M can also be calcium or barium. It could also be ammonium (NH₄). The R group consists of a perfluorinated hydrocarbyl group from 1 to 12 carbon atoms. Preferred are the C1 to C8. We unexpectedly discovered that the low molecular weight metal salt of perfluoroalkyl sulfonic acid are very soluble in the phosphate ester base stock whereas the potassium 3,3,4,4,5,5,6,6,7,7,8,8,8-Tridecafluorooctane-1-sulfonate (Zonyl® FS-62) was found essentially not soluble. This is illstrated in Examples 2 and 3 (Fluid E). . The said hydrocarbyl group can be linear or branched. A preferred example is metal salts of 1,1,2,2,3,3,4,4,4-nonafluorobutane-1-sulfonic acid. M is preferably an alkali metal and most preferably potassium.

[0012] The preferred examples of this invention are:

[0013] Potassium trifluoromethanesulfonate or potassium triflate;

[0014] Lithium trifluoromethanesulfonate or lithium triflate;

[0015] Potassium 1,1,2,2,2-pentafluoroethane-1-sulfonate-;

[0016] Potassium 1,1,2,2,3,3,3-heptafluoropropane-1-sulfonate-;

[0017] Potassium 1,1,2,2,3,3,4,4,4-nonafluorobutane-1-sulfonate-;

[0018] Potassium 1,1,2,2,3,3,4,4,5,5,5-undecafluoropentane-1-sulfonate-;

[0019] Potassium 1,1,2,2,3,3,4,4,5,5,6,6,6-tridecafluorohexane-1-sulfonate;

[0020] Potassium 1,1,2,2,3,3,4,4,5,5,6,6,7,7,7-pentadecafluoroheptane-1-sulfonate1; and

[0021] Potassium 1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-heptadecafluorooctane-1-sulfonate.

[0022] The foregoing additives are readily prepared by neutralization of the corresponding acid (i.e., a compound of the above formula except that M is H with an alkali metal hydroxide or quaternary ammonium hydroxide. Additives of the foregoing formula are also commercially available compounds.

[0023] The anti-erosion additive is incorporated in the phosphate ester basestock in an amount sufficient to enhance the anti-erosive properties of the fluid. Typically the addition comprises from about 0.01 wt % to about 0.5 wt % based on the weight of the basestock.

[0024] Phosphate ester base stocks used in this invention refer to organo-phosphate esters selected from trialkyl phosphate, dialkyl aryl phosphate, alkyl diaryl phosphate and triaryl phosphate that contain from 3 to 8, preferably from 4 to 5 carbon atoms. Suitable phosphate esters useful in the present invention include, for example, tri-n-butyl phosphate, tri-isobutyl phosphate, n-butyl di-isobutyl phosphate, di-isobutyl n-butyl phosphate, n-butyl diphenyl phosphate, isobutyl diphenyl phosphate, di-n-butyl phenyl phosphate, di-isobutyl phenyl phosphate, tri-n-pentyl phosphate, tri-isopentyl phosphate, triphenyl phosphate, isopropylated triphenyl phosphates, and butylated triphenyl phosphates. Preferably, the trialkyl phosphate esters are those of tri-n-butyl phosphate and tri-isobutyl phosphate.

[0025] The amounts of each type of phosphate ester in the hydraulic fluid can vary depending upon the type of phosphate ester involved. The amount of trialkyl phosphate in the base stock fluid comprises from about 10 wt % to about 100 wt % preferably from about 20 wt % to about 90 wt %. The amount of dialkyl aryl phosphate in the base stock fluid is typically from 0 wt % to 75 wt % preferably from 0 wt % to about 50 wt %. The amount of alkyl diaryl phosphate in the base stock fluid is typically from 0 wt % to 30 wt %, preferably from 0 wt % to 10 wt %. The amount of triaryl phosphate in the base stock fluid is typically from 0 wt % to 20 wt % and preferably from 0 wt % to 15 wt %.

[0026] The hydraulic fluids of this invention contain from 1 wt % to 20 wt % based on total weight composition of additives selected from one or more antioxidants, acid scavengers, VI improvers, rust inhibitors, defoamers. The use of those conventional additives provides satisfactory hydrolytic, oxidative stability and viscometric properties of the hydraulic fluid compositions under normal and severe conditions found in aircraft hydraulic systems.

[0027] Antioxidants useful in hydraulic fluid compositions in this invention include, for example, polyphenols, trialkylphenols and di (alkylphenyl) amines, examples of which include bis (3,5-di-tert-butyl-4-hydroxyphenyl) methane, 1,3,5-trimethyl-2,4,6-tris (3,5-di-tert-butyl-4-hydroxyphenyl) benzene, 2,6-di-tert-butyl-4-methylphenol, tetrakis (methylene (3,5-di-tert-butyl-4-hydroxyhydrocinnamate) methane, and di (n-octylphenyl) amine. Typical amounts for each type of antioxidants can be from about 0.1 wt % to 2 wt %.

[0028] Acid scavengers useful in hydraulic fluid compositions of this invention to neutralize phosphoric acid and dialkyl phosphoric acid produced from the hydrolysis and thermal degradation of the phosphate ester base stocks. Examples of acid scavengers include epoxy compounds such as epoxycyclo-hexane carboxylates. Typical amounts that can be used as acid scavenger can be from about 1 to about 10 wt % based on the total weight of hydraulic fluid.

EXAMPLE 1

[0029] This example illustrates the preparation of an additive of the present invention. To 100 g of sulfonic acid in 750 ml methanol is added with stirring enough of the corresponding alkali metal hydroxide to neutralize the acid. The methanol is removed by flushing the solution with nitrogen at 40° C. The product salt is then dried in a vacuum oven at 70-80° C. for 24 hours.

EXAMPLE 2

[0030] This example describes the functional fluids containing an alkali metal salt of a perfluoroalkyl sulfonate. The following functional fluids can be prepared by incorporating the particular salt into a tributyl phosphate, triarylphosphate base oil containing conventional VI improver, epoxide acid scavenger, antioxidant rust inhibitor and difoamer. FC-98 is currently used in commercial aviation phosphate esters and has been used in Fluids D and F as a comparison. FC-98 is a potassium perfluoroalkylsulfonate where R is a cyclohexanic ring. Examples of salts are shown in Table 1. TABLE 1 Fluid Additive Salt Concentration, wt % 1 Potassium 0.01 2 Lithium 0.5 3 Rubidium 0.01 4 Cesium 0.01 5 Potassium 0.5 6 Lithium 0.1 7 Quarternary 0.05 Ammonium 8 Ammonium 0.03

[0031] TABLE 2 Fluid A Fluid B Fluid C Fluid D Fluid E Fluid F Trialkyl 70-75 70-75 70-75 70-75 70-75 70-75 Phosphate, wt % Triaryl 10-12 10-12 10-12 10-12 10-12 10-12 Phosphates, wt % VI Improver, 5-7 5-7 5-7 5-7 5-7 5-7 wt % Additives, 8 8 8 8 8 8 wt % Anti-erosion, wt % Lithium triflate 0.05 0 0 0 0 0 Potassium 0 0.05 0 0 0 0 triflate Potassium 0 0 0.05 0 0 0 perfluorobutane sulfonate Potassium 0 0 0 0.05 0 0.025 perfluorooctane sulfonate (e.g., FC-98) Potassium 0 0 0 0 0.05 0 3,3,4,4,5,5,6, 6,7,7,8,8,8- tridecafluoro octane-1- sulfonate

EXAMPLE 3

[0032] This example illustrates the properties of the fluids from this invention. The anti-erosion additive provides electrical conductivity which is essential for the anti-erosion performance of the fluids. The example also shows an excellent solubility of the anti-erosion additive in the phosphate ester fluid. TABLE 3 Fluid A Fluid B Fluid C Fluid D Fluid E* Properties Appearance after 1 Clear Clear Clear Clear Clear week at −51 C. Potassium, wppm 0 101 52 34 6 Lithium, wppm 21 0 0 0 0 Conductivity, μS/cm 1.77 0.66 0.52 0.47 0.09

EXAMPLE 4

[0033] This example shows the effectiveness of the compositions of this invention to prevent erosion of the servo valve. The fluids used were those described in Example 2. The erosion rig test was performed according to the Boeing BMS 3-11L specifications. The tests were run at 225F and supplied pressure was maintained at 3000 psi at the exception of Fluid D which has been run at 275F. About 1000 ppm chlorine as 1,1,1-trichloroethane (methyl chloroform) was added after about 200 run hours. About 200 ppm chloride was used for Fluid D because it was run at higher temperature according to BMS 3-11L specifications. The servo valve edge was analyzed by SEM to confirm the presence of any servo valve edge electrochemical erosion. The results of the tests of the various functional fluids containing the perfluorinated alkylsulfonic metal salt of this invention are presented in Table 4. TABLE 4 Change Initial Final in Valve Leakage Leakage Run Leakage (Different Rate, Rate, Time, Rte, Servo Valve Valve) cc/min cc/min Hours cc/min Comments Fluid A 5 & 7 225 740 119 621 No damage but deposit on valve Fluid B 2 & 4 245 265 650 20 No damage, no deposit Fluid C 6 & 8 300 325 650 25 No damage, no deposit Fluid D 6 & 8 290 275 524 15 No damage, no deposit Fluid F 1 &3 300 280 663 20 No damage, no deposit 

What is claimed is:
 1. A hydraulic fluid composition, comprising: a phosphate ester basestock and a salt or mixture of salts represented by the formula RSO₃M where M is an alkali metal or ammonium; and R is a perfluorinated hydrocarbyl group having 1 to 12 carbon atoms, wherein the hydrocarbyl group is linear or branched.
 2. A hydraulic fluid composition according to claim 1, wherein the salt or mixture of salts comprises about 0.01 to about 0.5 wt % of the basestock; the perfluorinated hydrocarbyl group has 1 to 8 carbon atoms; and M is at least one member selected from the group consisting of lithium, sodium, potassium, and cesium.
 3. A hydraulic fluid composition according to claim 1 comprising, by weight percentage based on the total hydraulic fluid composition weight: a phosphate ester base fluid containing one or more trialkyl phosphate esters, wherein the alkyl groups of the phosphate ester contain 4 to 5 carbon atoms; 10 to 100 weight percent of trialkyl phosphates; 0 to 75 weight percent of dialkyl aryl phosphates, wherein the alkyl group contains from 4 to 5 carbon atoms and the aryl group is phenyl; 0 to 30 weight percent of alkyl diaryl phosphate, wherein the alkyl group contains from 4 to 5 carbon atoms and the aryl group is phenyl; 0 to 20 weight percent of alkylated triaryl phosphates, wherein the alkyl group is at least one member selected from the group consisting of isopropyl, n-butyl and tert-butyl; and 1 to 10 weight percent of an acid scavenger represented by the following formula:

where R₁ is H or a C₁ to C₄ alkyl group, x is 1 or 2, y is an integer of 1 to 4, and R₂ is a C₁ to C₄ alkyl group or a phenyl group.
 4. A hydraulic fluid composition according to claim 3, wherein the hydraulic fluid composition comprises, by weight percentage based on the total hydraulic fluid composition weight, 20 to 90 weight percent of the trialkyl phosphates.
 5. A hydraulic fluid composition according to claim 4, wherein M is an alkali metal.
 6. A hydraulic fluid composition according to claim 5 wherein M is potassium.
 7. A hydraulic fluid composition according to claim 1, wherein the formula comprises at least one member selected from the group consisting of: Potassium trifluoromethanesulfonate or potassium triflate; Lithium trifluoromethanesulfonate or lithium triflate; Potassium 1,1,2,2,2-pentafluoroethane-1-sulfonate-; Potassium 1,1,2,2,3,3,3-heptafluoropropane-1-sulfonate-; Potassium 1,1,2,2,3,3,4,4,4-nonafluorobutane-1-sulfonate-; Potassium 1,1,2,2,3,3,4,4,5,5,5-undecafluoropentane-1-sulfonate-; Potassium 1,1,2,2,3,3,4,4,5,5,6,6,6-tridecafluorohexane-1-sulfonate; Potassium 1,1,2,2,3,3,4,4,5,5,6,6,7,7,7-pentadecafluoroheptane- 1-sulfonate1; and Potassium 1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-heptadecafluorooctane-1-sulfonate.
 8. A hydraulic fluid composition according to claim 7, wherein the formula comprises potassium 1,1,2,2,3,3,4,4,4-nonafluorobutane-1-sulfonate- or potassium trifluoromethanesulfonate.
 9. A hydraulic fluid composition according to claim 1, containing: from 0.5 to 3 weight percent, based on the total hydraulic fluid composition weight, of an antioxidant or antioxidants mixture selected from the group consisting of: 2,6-di-tert-butyl-4-methyl phenol, tetrakis[methylene (3,5-di-tert-butyl-4-hydroxyhydrocinnamate)] methane, octylated diphenylamine, phenyl-α-naphthylamine, diphenylamine, ditolylamine, 4,4′-diaminodiphenylamine, and mixtures thereof.
 10. A hydraulic fluid composition according to claim 9, wherein the antioxidant or antioxidants mixture is 0.5 to 2 weight percent, based on the total hydraulic fluid composition weight.
 11. A hydraulic fluid composition according to claim 1 containing from about 3 to about 10 weight percent, based on the total hydraulic fluid composition weight, of a viscosity index improver, comprising: an acrylate or methacrylate ester polymer where the repeating units substantially comprise n-hexyl; and idodecyl acrylate or methacrylate, wherein at least 95% by weight of the polymer has a molecular weight between about 50,000 and 900,000.
 12. A hydraulic fluid composition according to claim 11, containing from 4 to 8 weight percent, based on the total hydraulic fluid composition weight, of the viscosity index improver.
 13. A method of inhibiting the erosion tendency of a phosphate ester based fluid comprising incorporating in the fluid from 0.01 to about 0.5 wt % of a salt or mixture of salts represented by the formula RSO₃M where M is an alkali metal or ammonium; and R is a perfluorinated hydrocarbyl group with 1 to 12 carbon atoms, wherein the hydrocarbyl group can be linear or branched.
 14. A method according to claim 13 wherein M is an alkali metal and the hydrocarbyl group has 1 to 8 carbon atoms. 